This invention relates to solid state relays and, more particularly, to circuits for providing power to multiple loads in a non-overlapping fashion in order to simulate the break-before-make contact configuration of an electromechanical relay.
Over the years, a wide variety of solid state relays have been developed to control the application of power to a load. Recently, metal oxide semiconductor field effect power transistors (MOSFETs) have been used as output switching devices in these types of circuits. Some of these circuits employ light emitting diodes optically coupled to photovoltaic diode arrays which control the MOSFET. Examples of circuits of this type are disclosed in copending United States patent application Ser. No. 627,808, filed July 5, 1984, entitled Current Overload Protected Solid State Relay, and assigned to the assignee of the present invention.
For the most part, prior art solid state relay circuits are limited to a single output switch closure in response to the application of a control signal. This configuration is analogous to an electromechanical relay having a single-pole-single-throw, normally open contact.
While it is desirable to provide a solid state relay having an output switch configuration analogous to that of a single-pole-double-throw, break-before-make mechanical contact, such devices have been difficult to construct. One of the controlling factors in the design of a solid state relay of this type is the response time of the output switching device. This is so for the following reasons.
In a break-before-make contact configuration, one contact pair opens before the other contact pair closes, so that there is no overlap of switch closures. In order to implement this configuration using two solid state switches, it is necessary that one of the switches be fully off before the other switch is turned on.
To meet the above criteria when using MOSFETs as output switching devices poses a particularly difficult problem. This is so because the high gate to source resistance and capacitance associated with MOSFETs generally yields devices in which the turn-off time is substantially longer than the turn-on time. This characteristic leads to the overlapping of switch closures when two MOSFETs are operated in alternating fashion.
Accordingly, it is an object of the present invention to provide a new and improved solid state relay.
It is another object of the invention to provide a solid state relay having multiple output switching circuits.
It is yet another object of the invention to provide a solid state relay having multiple output switching circuits which operate without overlapping switch closures.